This invention relates to a color wheel for use in an optical system of a multicolor image projection unit.
The color wheel is provided in an optical path in the optical system of the multicolor image projection unit, and is used to provide a periodical change in color. More specifically, the color wheel is shaped like a circular disk cyclotomically divided into sectors consisting of filters of different kinds, each capable of selectively transmitting a specific wavelength region of light in white light incident on. As the color wheel rapidly spins, each filter alternately comes in the optical path through which rays of light from the light source pass, causing the rays of light to periodically change in color. The rays of light that have passed through the color wheel and got into an image converter are projected on a screen so that a series of multicolor images as thus projected on the screen can be integrated in the eyes of a viewer, so as to generate full-color images. The color wheel as above is used for example in a sequential color display unit, which includes: a liquid crystal projector and a DMD (digital micromirror device) projector having an optical system such as a DLP (digital light processing).
A color wheel as disclosed in JP 2001-337390 A is, for example, shaped like a circular disk in its entirety with a plurality of color sectors; i.e., fan-like color filter segments are arranged on the circular disk, or so arranged as to assume a discal shape.
However, this arrangement requires precisely cutting out such fan-like color filter segments, and finishing the fan-like color filter segments into a discal shape by bonding the same with an adhesive or the like. Accordingly, boundaries between the color filter segments that could possibly involve an error in fabrication would inevitably cause various problems in the resulting color wheel, such as noise produced in the rays of light passing through the boundaries, reduced rotation strength, unbalanced rotation, wind noise, and the like, to which such a fine adjustment must be made as would disadvantageously require skills, time, and costs.
Besides, disclosed in U.S. Pat. No. 5,711,889, as an example of methods of fabricating the color wheel, is a method for making a three-color filter array on one disk utilizing a photolithographic technique.
However, assuming that the method of making the filter array includes one cycle of three processes for red (R), green (G) and blue (B), and that a central angle formed between two radii of each sector of the resultant color filter segments is 120 degrees, a photomask capable of shielding a sector having a central angle of 240 degrees (in a case where a positive resist is used) or of shielding a sector having a central angle of 120 degrees (in a case where a negative resist is used) should be employed for an exposure step of each process and an alignment of the photomask should be made three times, i.e., once for each color (R, G and B). In this instance, based on a premise that errors could be involved in making such an alignment, the color filter segments of RGB-transmittable film would overlap with each other or a gap would be left therebetween; resultant overlap or gap could disadvantageously cause blur or flicker in images produced with the color wheel. It goes without saying that making every effort to minimize an error in alignment of the photomask would successfully provide a high-precision color wheel capable of producing images with less blur and flicker that could otherwise be caused by the overlap or gap between the color filter segments of selective color-transmittable film. However, to achieve this, very expensive facilities for a patterning process would have to be installed; consequently this approach could not be deemed to offer practical solutions.
Moreover, a normal process of the photolithographic technique in which an alignment of the photomask is made repeatedly for each color (R, G and B) would require the processes of photomask alignment and exposure to be performed three times, i.e., once for each color (R, G and B), and thus would disadvantageously need enormous labors and energy, prolong the process time, and reduce the process speed, for fabricating a color wheel.
It is an exemplified general object of the present invention to provide a color wheel fabrication method that can totally eliminate the above-described disadvantages in the conventional approaches of bonding fan-like color filter segments with an adhesive or the like and of making a three-color filter array on one disk utilizing a photolithographic technique. Another exemplified and more specific object of the present invention is to provide a color wheel fabrication method that is superior to the conventional approaches in various aspects of features such as cost, accuracy and ease of fabrication.